1. Field of the Invention
The invention relates to surgical computer systems, including computer program products, and methods for cumulative buffering for surface imaging.
2. Description of Related Art
The use of computers and computerized equipment is becoming more prevalent during surgical procedures. If the position of the surgical instrument is tracked by a surgical computer system, a computer can generate a visualization of the surgical instrument's position relative to the patient's anatomy. A visualization is, for example, a simulated three-dimensional image displayed on a computer screen. The visualization can be based on images of the patient's actual anatomy (such as CT scans). If the surgical instrument is removing tissue from the anatomy, e.g., a burr removing bone tissue in preparation to receive an implant, the visualization can be a simulated image of the anatomy showing the removed tissue based on the position of the instrument. Such an exemplary surgical computer system is described in detail, for example, in U.S. Patent Publication 2006/0142657, published Jun. 29, 2006, which is hereby incorporated by reference herein in its entirety.
The exemplary surgical computer system can use, for example, the “isovol” technique, or iso-volume technique. Using this technique, the volume of the bone area being worked on and the volume of the burr being used are calculated. The volume of the burr is then subtracted from the volume of the bone. This subtraction is performed by a CPU of a computer. From this subtraction process, a surface model of the bone area is calculated, again using the CPU. A graphics card generates the visualization based on the surface model and it is displayed on the screen. When the burr is moved to a new location, the burr volume is subtracted from the bone area at the burr's new position. A new surface model is then calculated. Because the old surface model is no longer accurate, it is cleared from the buffer and the new surface model is buffered. This is sometimes referred to as the “clear and display” approach. This process is continuously repeated as the burr is moved. Because this processing is done in real time, as the burr is moving, the resolution is low to allow for the necessary computations.
Another technique that is used for three-dimensional modeling is constructive solid geometry (CSG). CSG is typically used in geometric modeling, such as a Computer Aided Design (CAD) package for modeling components. In CSG techniques, complex shapes are built from simple shapes by volumetric Boolean operations, i.e. union, intersection, and subtraction. A complex shape is specified by a CSG expression, which is commonly stored as a CSG tree whose leaf nodes represent basic shapes (primitives such as sphere, cylinder, and box) and inner nodes denote Boolean operations. See, e.g., the examples shown at the constructive solid geometry web page at wikipedia, the online encyclopedia.
Image-based CSG algorithms are a category of algorithms for z-buffer graphics hardware that generate “just an image” of a CSG shape without calculating a description of the final object geometry. Image-based CSG takes advantage of the hardware acceleration in a video card (also referred to as a graphics card), which can improve rendering speed. CSG also can produce less visual artifacts than a possibly approximated 3D geometry.
The CSG techniques have better resolution than the isovol technique and CSG can allocate its visualization tasks to a video card, and thus increase CPU performance and resolution. However, as the burr is moved, the number of leaf nodes (primitives) on CSG tree grows larger and larger, and this makes the conventional “clear and display” approach algorithm described above not feasible for sophisticated tasks such as visualizing a bone preparation process.